This invention relates to offset printing presses, and particularly to a web-fed offset printing unit capable of changing images to be printed on a web of paper or like material with the web kept running during such image changes, and to a printing machine incorporating a plurality of such units.
Japanese Unexamined Patent Publications Nos. 8-197826, 8-207233, and 9-314813 are hereby cited as teaching web-fed offset printing presses capable of changing printing images without stopping the web. The first cited prior art machine (8-197826) has a plurality of printing units in alignment along the path of the web, the printing units having the same or, different printing plates for printing one or both sides of the web. A required set of printing plates are first clamped around the plate cylinders of all the printing units. The printing units are selectively put to use for successively printing the images on the web in a required sequence. For each image change the plate cylinder or cylinders of the printing unit to be set out of operation are moved away from the associated blanket cylinder or cylinders, and the plate cylinder or cylinders of a desired printing unit that has been out of operation are actuated into contact with the associated blanket cylinder or cylinders for printing a new image on the web.
The second known printing press (8-207233) is analogous with the first cited machine in that a plurality of printing units are aligned along the web path and selectively used for printing different images on the continuously running web in a prescribed sequence. Only the blanket cylinder of the printing unit that has been in operation till an image change is moved out of contact with the web, and that of the unit having the next image to be printed is moved into contact with the running web.
This second reference discloses a variation featuring a blanket cylinder that is twice as long in circumference as each plate cylinder, in order that the different images on two different plate cylinders may be concurrently printed on the web through one and the same blanket cylinder.
In this variation, too, image changes are accomplished by selective use of the different printing units. The two plate cylinders of a printing unit to be put out of operation are moved out of contact with the blanket cylinder, and, in a printing unit that has been out of operation, the two plate cylinders bearing the new plates are jointly moved into contact with the blanket cylinder of that unit. A change is thus made concurrently from two images to two others. No plate cylinders are of course in contact with the blanket cylinder on the nonprinting side of the web.
The third mentioned reference discloses two modes of carrying out the invention. In one such mode two sets of a plate cylinder and a blanket cylinder are employed in conjunction with a single impression cylinder around which the web is threaded. A change from one set to the other, and hence from one image to the other, is achieved without stopping the web. This first mode also permits printing either directly from the plate cylinders or indirectly through the blanket cylinders. Therefore, for an image change, the blanket cylinder or the plate cylinder that has been in operation is moved away from the web, and the other blanket cylinder or plate cylinder is driven into printing engagement with the web.
The second mode of the third reference is directed to an offset perfecting press, comprising a plurality of printing units each having a pair of blanket cylinders on both sides of the web and a pair of plate cylinders each in contact with one blanket cylinder. An image conversion is accomplished upon retraction of the plate cylinders of one printing unit that has been in operation away from the blanket cylinders of that unit, followed by the movement of the plate cylinders of some other desired printing unit into contact with the blanket cylinders of that other printing unit. All the blanket cylinders are synchronously rotated from a common drive shaft.
In changing images by any of the methods set forth hereinbefore the peripheral speed of the plate cylinder, or of both plate cylinder and blanket cylinder, to be moved into printing engagement with the web must be preadjusted to the traveling speed of the web at the moment of image change. The aforementioned prior art machines are all equipped for such preadjustment.
All these conventional machines are objectionable, however, for more reasons than one. First of all, they are all alike in having a plurality of printing units, each comprising at least a blanket cylinder and one or two plate cylinders, aligned along the path of the web, with a required spacing from one unit to the next. Such units include not only those that must be put to joint operation for the printing job but an additional unit or units for image changes. The row of printing units have rendered the machines inordinately long in the traveling direction of the web, demanding large installation spaces and making the presses very expensive in construction.
The stacking of the multiple printing units would provide no fundamental solution to the problem in consideration of the excessive height of the resulting machine. Such a machine would, moreover, require labor at heights and so would be poor in operability, maintenance and management.
Another objection arises from the fact that the plate cylinder or cylinders are out of contact with the blanket cylinder in any printing unit that is out of operation. That blanket cylinder is not dampened during such time. Consequently, when any printing unit is set out of operation, a varied spacing has been created between those damping positions on the web where images are being offset printed from the, moistened blanket cylinders. The web has been susceptible to different degrees of lateral expansion, or fanout, as it traverses the successive printing units, resulting, in the case of a multiple printing press, in the nonregistration of an image that has been newly printed on the web and the images printed by the units lying upstream and downstream, respectively, of the newly printing unit. Large amounts of paper have been wasted because of such unregistered image production.
With the plate cylinder or cylinders kept out of contact with the blanket cylinder in any nonprinting unit as above, the unmoistened blanket may be left in contact with the web for an extended period of time. Thereupon, as is familiar to the printing engineers, large amounts of paper fibers and particles have frequently transferred from web to blanket. Additionally, if an image or images are being printed upstream of the nonprinting unit, the fresh ink on the web has been easy to stain and smear the nonprinting blanket. The ink has been prone to be retransferred from the blanket back to the web, out of place of course, to the detriment of the printing quality.